Although histamine is localized in regions that represent phylogenetically older parts of mammalian brain, there has been no systematic study of its occurrence in brain of primitive vertebrates. Histamine was found in brain of all twelve vertebrate species tested. Species which did not have a cerebral cortex (neopallium) or a well-developed cerebellum, such as amphibia, fish and hagfish, had higher cerebral histamine levels than mammalian species, even though histamine was present in only trace (less than 0.1 microgram/g tissue) amounts in most peripheral tissues (an exception was gastric mucosa) of these species. In dogfish, histamine was present throughout the brain stem and spinal cord but was mostly concentrated in the midbrain region. A large proportion of this histamine could be recovered in washed synaptosomal preparations. Histamine methyltransferase and diamine oxidase activity was present in brain of all aquatic species, whereas only methyltransferase was present in mammalian species. The studies add further support to the idea that histamine is a neurotransmitter and suggest that histaminergic neurones innervate primitive regions of the brain.